This invention relates to electrophotographic apparatus such as copiers and photoprinters.
There are many reasons for which quality of image formed by an electrophotographic apparatus changes unreliably. With selenium-based photoreceptors, for example, it is well known that the rate of change in darkness varies, depending on the length of their rest period and that it is because the charge trapped in the photoreceptor layer is gradually released during a rest period. Such phenomena cause big changes in the image density when the electrophotographic apparatus is restarted and adversely affect the image quality. FIG. 1 shows by way of an example the change in the surface voltage of a photoreceptor comprising As.sub.2 Se.sub.3 when copies are made continuously by a copier using such a photoreceptor after this copier is left unused for one hour in a ready condition. As can be seen in FIG. 1, the surface voltage is at the highest at the end of a rest period, both at dark and bright areas, dropping rapidly as soon as copying is started but stabilizing after a certain number of copies have be produced. The rate of this voltage drop varies, depending mainly on the length of the preceding rest period and also on the individual characteristics of the photoreceptor. In general, the longer the rest period, the greater is the rate of voltage drop. FIG. 2 shows the relationship between the cumulative copy count and the copy density in this situation. Since copy density is proportional to surface voltage of the photoreceptor, the curves in FIG. 2 are similar to those in FIG. 1, indicating that the density is the greatest on the first copy sheet but drops rapidly thereafter, stabilizing after a certain number of copies have been made. For example, there is a big difference in density between the first copy and the 200th copy and this means that the copy quality is not reliably uniform.
In view of the above, Japanese Patent Publication Tokko No. 60-41351, for example, disclosed a copier with the capability of controlling the charging of the photoreceptor, its exposure, the developer bias voltage and the toner supply rate according to the length of the preceding rest period in order to uniformly adjust the copy density. As explained above, however, the surface voltage which determines the copy density depends not only on the length of rest period but also on the fluctuations in the individual characteristics of the photoreceptor and other factors. Thus, uniform copy quality cannot be obtained if the copier operation is controlled only on the basis of the length of preceding rest period.
With electrophotographic apparatus using a two-component developing agent (for example, toner and a ferrite carrier) for forming a magnetic brush for its developing process, the phenomenon of so-called fogginess tends to appear when the apparatus are restarted after a rest period during which power is switched off. Because toner in the developing tank is not sufficiently charged as a whole when the apparatus is restarted, some toner particles are very weakly charged or even oppositely charged and they may come to be deposited on the surface of the photoreceptor independently of the Coulomb force and merely by mechanical contact. Toner comes to be weakly charged because of electrostatic discharge inside the developing tank during rest periods. In addition, since the apparent density of developing agent changes after it is left unused for a long time, the control sensor of the apparatus is activated when the apparatus is restarted such that an excessive amount of uncharged toner is freshly supplied into the developing tank, causing a drop in the overall level of toner charge. It now goes without saying that fogginess makes it difficult to read the hard copies made by the apparatus, or generally affects the image quality adversely.
A method of preventing fogginess in copiers and the like was suggested, for example, in Japanese Patent Publication Tokkai No. 50-46334. According to this method, the bias voltage applied for development is increased gradually from the time developing agent is replaced as the cumulative copy count increases such that the effect of deterioration of the developing agent can be compensated for. Although this method can prevent the occurrence of fogginess caused by the type of deterioration advancing slowly over a long period of time, it cannot prevent fogginess which appears when the power switch is turned on and the copier is restarted because the toner charge is actually dropped after each rest period.
Another method was disclosed in Japanese Utility Model 57-54163 for preventing adverse effects on image quality due to deterioration of developing agent. According to this method, use is made of a device for adjusting the speed of development on the basis of an output signal from a sensor for detecting toner concentration or a sensor for detecting developing capability of the developing agent. When such a sensor outputs a signal indicative of a deteriorated condition of the developing agent, however, a conventional device of this type increases the rotational speed of the magnet roller (for development) so as to decrease the speed of development. This is because, if iron particles (of Fe.sub.2 O.sub.3) are used as carrier, the electroconductivity of the carrier particles drops as toner is attached to their surfaces while toner charge is increased and the image density decreases. If ferrite is used as carrier, however, the resin coating on the ferrite carrier becomes removed gradually and the electroconductivity of the ferrite carrier itself increases, causing the toner charge to drop. Thus, if such a compensating device as disclosed above is used with a copier using a developing agent having ferrite as carrier, the copy density increases instead to adversely affect the image quality.